Hmmm.... well let's start off:
Welcome to FT!

First, let's talk what a legal connection is. A legal connection is the minimum connecting time from one type of flight to another. A legal connection between an int'l flight and a domestic flight will probably be longer than one from a domestic to a domestic. These practices are set by airports/airlines. In the case described above (int'l=>domestic), a legal connection will not give you enough time to connect with baggage, especially if the two airlines don't have an interline agreement (sounds like CapeAir does not have one with US, or you got a bad Cape Air agent). In the future, even if you have a legal connection, try to book a longer one, so that you don't miss your flight. If you did not have luggage, chances are you would have made it.

US Airways is obliged to get you from PHL to your destination. If you miss your flight due to a late incoming flight, they are responsible for getting you to your destination on another flight. It sounds like that is what happened at the end, so they fulfilled their end of the contract.

Expedia cannot do anything while you are mid-trip, as the ticket would be handled by US Airways.

The MCT are not publically available anywhere, except from a travel agent who has access to a GDS. Chances are your connection was legal.

The fact that the flight was 10 min late makes it "on-time" per FAA regulations.

Although if we stick to the hard rules of the matter, you are not due any compensation from anyone, if I were you, I'd write to everyone, briefly explain the situation and see what you get.

Next time, book a longer connection, as 51 mins is not enough time to connect with luggage.